H^EMOPHILUS AND ASSOCIATED ORGANISMS 281
The optimum temperature is 37° C. and aerobic conditions are
required; the influenza bacillus is considered to be a strict
aerobe.
In fluid media, a slight general turbidity is usually formed.
The thread-like forms tend to give a flocculent deposit with little
turbidity.
On solid media, minute, smooth, transparent, circular colonies
are formed in 24 hours. Atypical strains, such as those showing
threads, give colonies that are more opaque and have a bluish
iridescence. Growth becomes more marked on further incubation
and the colonial appearance tends to become irregular. When
grown on a blood-plate with a Staphylococcus the colonies of
H. influenzas, growing around a staphylococcal colony, are usually
larger than the discrete influenzal colonies. This phenomenon is
termed " satellitism " and is due to the synthesis by the Staphylo-
coccus of the V factor, which diffuses into the surrounding
medium ; this phenomenon is given on agar by H. parainfluenzce
as these organisms require the V, but not the X, factor.
Resistance. The influenza bacillus is readily destroyed by
bactericidal agents. It is killed by heat at 60° C. in 5 minutes.
It is difficult to conserve ; cultures should be kept at 37° C. and
subcultures must be made every 5-7 days.
Biochemical Activity. The biochemical properties of the
influenza bacillus have been inadequately studied mainly on
account of its growth requirements. The difficulty in obtaining
growth is also doubtless responsible for the discordant results
obtained by various workers.
In the case of the fermentation reactions discrepancies have
arisen, but many workers agree that acid, but not gas, is produced
from glucose, maltose and laevulose by most strains. The
proteolytic activity is very slight, but many strains are able to
produce indole. Indole-formation appears to be a stable character,
and this property has been used as the criterion for the sub-
division of the influenza bacilli into two groups—the indole
producers and the non-indole producers. The non-indole pro-
ducing strains are often atypical, haemolytic and avirulent to
mice.
Haemolysis is shown by some strains when grown on blood-agar
plates. These hsemolytic strains usually do not require the X
factor, they fail to form indole, and, on morphological examina-
tion, occur often as threads or long rods ; they are H.
fluenzce.